The invention refers to the fields of technology herein laser diodes are utilized, for example to reproduction technology, and is directed to a method and to a circuit arrangement for driving a laser diode in a recording device such as a laser printer or laser recorder.
In such a recording device, the light output by the laser diode and modulated with the information to be recorded is shaped into a recording beam with optical means and is deflected point-by-point and line-byline across a recording medium with a deflection system during recording.
In most cases, a constant current source is employed for driving the laser diode. The constant current source generates a constant driver current that is modulated by an image signal that contains the information to be recorded and is supplied to the laser diode.
The laser diode works in switched mode fashion for recording line information, wherein the driver current modulated by a two-level image signal switches the laser diode on and off.
In order to achieve a high recording quality in the recording of line information, the laser diode must switch quickly and the light level output by the laser diode must be constant in the on-time intervals in order to achieve a uniform illumination.
Based on its very nature, the laser diode does not satisfy the demand for a constant light level in the on-time intervals.
The output light power is temperature dependent, namely such that the light power decreases with rising operating temperature of the laser diode.
The gradual rise in the operating temperature in the substrate of the laser diode and the drop of the output light power caused as a result thereof can in fact be compensated by regulating the housing temperature; in a modulation or, switched mode, however, the laser diode still has a dynamic temperature effect. The cause of the dynamic temperature affect is the temperature change of the laser transition in the chip dependent on the modulation or, on the image signal that controls the modulation. Due to this temperature dependency of the laser transition, a temperature difference between substrate and laser transition continues to exist even when the housing temperature is regulated and an internal temperature compensation process is the consequence. Given a laser diode working in switched mode, this inner temperature compensation process leads to a variation of the output light power dependent on the image signal in such a way that the output light power rises above the nominal level in the respective turn-on time and then only gradually reaches the nominal level within the individual on-time intervals. Due to this effect, a disturbing lag effect that has a considerable influence on the recording quality arises on the recording medium when recording line information.
Various measures for correcting the light power output of a laser diode are already known.
For example, it has already been proposed to measure the light power output by the laser diode within the respective time interval required for the recording of a line and to control the light power via the driver current dependent on the measured result.
GB Patent 21 01 851 likewise already discloses that the output light power be measured with a photodiode (monitor diode) integrated in the laser diode within or outside the time interval required for the recording of a line, to calculate correction values from the respectively measured light power, to intermediately store the correction values line-byline in sample-and-hold circuits, and to control the light power via the driver current dependent on the stored correction values.
A regulation for correcting the light power output has the disadvantage that it is involved and that stability problems arise due to the control loops, and the switching speed of the laser diodes is reduced.
It is therefore an object of the invention to specify a method and a circuit arrangement for driving a laser diode with which a simple correction of the output light power and an improved switching behavior are achieved.
According to the method of the invention for driving a laser diode, the laser diode is charged by a driver current which defines an output light power, the output light power being dependent upon and dropping with a rise of internal temperature of the laser diode. For compensating the temperature-dependent drop in light power, the laser diode is driven by a voltage generator having a variable internal resistance R.sub.i. The internal resistance is calculated according to the equation: ##EQU1## wherein .sigma. is the efficiency of the laser diode, T.sub.KU is a temperature coefficient based on an on-state voltage of the laser diode, T.sub.kp is a temperature coefficient based on a power of the laser diode, and R.sub.Di is an internal resistance of the laser diode, a rise of the driver current given rising temperature of the laser diode being limited by the internal resistance R.sub.i such that a rise in the output light power caused by a rise of the driver current compensates the temperature-dependent drop in light power.
As a result of these techniques, it is particularly a correction of the disturbing lag effect that can be achieved given employment of the laser diodes in recording devices without deteriorating the switching speed and a good recording quality given high recording speed can be achieved overall.
The invention shall be set forth in greater detail below with reference to FIGS. 1 through 3.